SIGNIFICANCE OF THE SYMBOL pH 153 



Measurement of hydrogen-ion concentration. The concen- 

 tration of hydrogen ions is expressed numerically in terms of a 

 normal solution. A normal hydrogen-ion solution contains 1 

 gram of hydrogen ions or the equivalent per liter. Normal solu- 

 tions are therefore made up on the basis of molecular weight to 

 secure a solution containing 1 gram of hydrogen or the equivalent 

 per liter. The dissociation constant of a IN solution of the 

 strongest acid, HC1, at 25° C is essentially 1. The dissociation 

 constant of the weakest acid, pure water, has been determined to 

 be 1/10,000,000 N, which constitutes neutrality. It follows there- 

 fore that the dissociation constants of all other acids are fractions 

 that range between these extremes. 



The dissociation of pure water at 25° C, if expressed formally, 

 would be written 



[H+] X [OH-] _ 

 [HOH] ~ Au " 



If*the concentration of hydrogen ions in water is 1/10,000,000 

 gram (or 10 ~ 7 ), if expressed logarithmically) and water is neutral, 

 then the concentration of hydroxy 1 ions is also 1/10,000,000 gram 

 (or 10~ 7 ). The number of molecules of water dissociated is 

 so small in comparison with the total number that [HOH] may 

 be considered unity and omitted, making the formal equation 

 [H+] x [OH-] = K w , or [H+] (10" 7 ) X [OH~] (10~ 7 ) = 

 K w (lO- 1 ^). 



Significance of the symbol pH. Since the hydrogen-ion con- 

 centration of a solution is, with few exceptions, a fraction of the 



normal, it may be expressed as jtt+i, that is, the reciprocal of 



[H+ ] . By use of the reciprocal the negative exponent is avoided. 

 The symbol pH is therefore used to designate the logarithm of the 

 reciprocal of the hydrogen-ion concentration. The hydrogen-ion 

 concentration of pure water, for example, is 1/1*0,000,000 N. 

 Expressed otherwise, 



[H+] = 1 X 10- 7 , 

 or 



r+ 



or 



log [H+] = -7, 

 - log [H+] = +7, 



or j 



log jjpj = 7, 



or 



pH = 7. 



